As reported, for example, in Prota, Progress in the Chemistry of Melanins and Related Metabolites, Med. Res. Reviews, 8:525-56 (1988), melanins are naturally occurring pigments present in hair and skin. In humans biosynthesis takes place in tyrosinase containing melanocytes. The tyrosinase enzyme catalyzes the hydroxylation of tyrosine to dopa and its subsequent oxidation to dopachrome. Once formed, dopachrome undergoes a series of complex reactions in the formation of eumelanins and phaeomelanins. Eumelanins provide black and brown pigments, and are formed by oxidative polymerization of 5,6-dihydroxyindole derived biogenetically during the melanogenesis. On the other hand, phaeomelanins provide yellow to reddish brown pigmentation to hair, and are formed by oxidative polymerization of cystein-S-yl-dopas via 1,4-benzothiazine intermediates.
Synthetic 5,6-dihydroxyindole (DHI) has been disclosed in the prior art for use in hair and skin dyeing. For example, U.S. Pat. No. 2,934,396 to Charle discloses a process for dyeing hair by contacting hair with an aqueous solution of DHI having a pH of at most 7 for 5 to 60 minutes, followed by an application of an aqueous solution capable of inducing oxidation and/or polymerization of DHI.
Dopa and dopamine are disclosed as hair dyeing precursors in the process of Herlihy, U.S. Pat. No. 4,746,322, wherein the aqueous hair dyeing composition comprises said precursor, an organic compound to assist dye dispersion and an iodate or periodate. The dopa or dopamine dye precursor is present in the aqueous hair dye composition in an amount of from about 1 to about 100 mg/ml, preferably from about 5 to about 25 mg/ml. Dopamine is preferred, according to Herlihy. The iodate or periodate is present in the composition at a concentration of 1 to about 50 mg/ml, while the dispersing agent is present in an amount of from about 0.1 to 30% (wt./vol.). Optionally, a color modifier can be incorporated into the aqueous composition of Herlihy, at a level of from about 0.1 to about 10 mg/ml. pH may be maintained between about 3 to about 7 by incorporation of an effective amount of a buffer. According to Herlihy, the above-described aqueous compositions disperse the dye on the hair shaft "with little or no penetration into the hair shaft." Column 2, lines 56-58.
The prior art fails to provide a commercially feasible process for effectively, permanently dyeing hair using dopa as a starting reagent. It is believed this failing is attributable to an inability of the prior art processes in making 5,6-dihydroxyindole available on the hair at concentrations suitable for its diffusion into the hair, for subsequent conversion to nondiffusable melanin, as further explained in detail below.
Indeed, the inability to provide an inexpensive yet effective process for dyeing hair with a melanin precursor has prevented use of melanogenesis in the commercial dyeing of hair.
Notwithstanding such inability, interest in melanogenesis to dye hair remains quite high. This is because synthetic melanin pigments provide an exceptionally natural-looking brown or black color. Moreover, they are not irritating to the skin. Nor are they mutagenic. Accordingly, the hair dyeing art has long awaited the present invention--a process for permanently dyeing hair with synthetic melanins that is economically viable--and represents a major advance in the art.
It has now been found, quite surprisingly, that an aqueous hair dyeing process wherein an effective melanin-forming hair dyeing amount of 5,6-dihydroxyindole is generated during the reaction of dopa with an oxidant can be practiced inexpensively and under commercially feasible conditions, to achieve a permanent hair color.